Iodine is a substance that readily becomes an anion. It thus is one of the substances that is suitably used as an active material of positive electrode. Iodine is known to form charge transfer complexes with various organic compounds. The charge transfer complexes of iodine are composed of iodine (acceptor) and various electron donating organic compounds (organic donor component). The known electron donating compounds include the heterocyclic compounds such as phenothiazine and carbazole and the polyaromatic compounds such as pyrene and perylene and such organic polymers as poly-2-vinylpyridine, polyethylene, polyacetylene, poly-p-phenylene, polythienylene, polypyrrole, polyaniline, polyvinyl alcohol, polypropylene and polystyrene.
It was further found by us that such organic polymers as the urea/formaldehyde resins, polyurethane, polyacrylonitrile, polyamides, poly(meth)acrylamide and polyethers are also effective as the organic donor component.
In all instances there is seen a great rise in the conductivity of these charge transfer complexes as compared with that of the original starting simple substances.
Of these complexes, the poly-2-vinylpyridine/iodine complex is already being used as the positive electrode materials of lithium cells. On the other hand, in recent years extensive development of such compounds as polyacetylene, polypyrrole, polyaniline, poly-p-phenylene and polythienylene, as conductive compounds, is being carried out. It is known that there is an enhancement in the conductivity of these compounds by doping them with iodine. There are however various drawbacks such as that such compounds being easily oxidized by oxygen are not stable, that their processability is poor, etc.
The iodine charge transfer complex (hereinafter referred to as iodine complex or simply as complex adduct) brings about a great enhancement in the conductivity of such polymers as the polyamides, polyvinyl alcohol, etc., which per se are insulators. The following problems, however, arise when these iodine complexes are to be applied to their use as a conductor. In first place, the conductivity of the iodine complex or complex adduct shows a great fluctuation depending upon the amount of iodine contained, especially in the range where the content is small, with the consequence that difficulty is experienced in its control. Secondly, a considerably large amount of iodine, for example 70 to 80% by weight, must be incorporated in the polymer for obtaining one having a high conductivity. It is not necessarily an easy matter to incorporate the iodine in such a high concentration. Furthermore, a complex adduct in which great fluctuations take place in the conductivity depending upon the content of iodine is objectionable for such applications where fluctuations in the content of iodine inevitably take place at the time of their use. For example, take the case where the complex adduct is used as positive electrode materials. Since the iodine would become separated from the complex adduct as the discharge proceeds, the positive electrode materials would gradually lose its conductivity. As a consequence, a marked increase takes place in the resistance of the positive electrode materials as the discharge proceeds, and this brings about a great reduction in the electromotive force of the cell.
On the other hand, when positive electrode materials composed of this charge transfer complex is used in a secondary cell, a major part of the iodine becomes separated from the positive electrode materials as a result of the discharge. Hence, the positive electrode material becomes an insulator. This becomes a crucial defect in the case of a secondary cell, for it becomes impossible to recharge it. This is a problem that inevitably occurs in the case where the iodine complex of the polymer used is one in which the polymer is by nature an insulator, such as the polyamides, polyacrylonitrile, polyvinyl alcohol, polyethers and poly-2-vinylpyridine.
Extensive researches by us with the view of overcoming these shortcomings of the charge transfer complexes led to the discovery that the foregoing shortcomings could be overcome by incorporating a carbon in the complex. The present invention was thus perfected.